We started the game with three classes capable of tanking. We're up to five. Along the way, tanking has changed and changed again, until its modern implementation barely even resembles what we were doing back in the days of ten or fifteen person UBRS groups - tanking today has a host of mob control abilities in order to allow them to more effectively control groups of adds, tools for mobility and is based around actively reducing incoming damage in a way it simply wasn't years before. Now, consider this - how does the game itself change in order to challenge the modern tank? What does it do to demand they play to their best? Encounters of the past wouldn't even make a modern tank blink - what challenge would Garr pose to today's tank, for example? A bunch of adds? Bring it. So design has to take these new tanking modes and abilities into account and provide new ways to give them difficult encounters... and these encounters thus create, in their turn, the new tank of the future.
Meanwhile, all of this work also changes healers, and DPS, and how they change also changes encounter design. New abilities beget new difficulties that in their turn beget new abilities. New and better gear means that encounters (even out in the world) must be designed to meet them. Leveling content in Mists of Pandaria had to offer challenges to people who were in raiding gear, which meant that players who were not in said gear had to have a way to catch up. This has been true since The Burning Crusade and it is the reason that we went from straight percentages on gear to the rating system for stats like crit - the complexity here is in the service of making players want new gear, since their old gear will grow weaker even as they are leveling. It's a counter-intuitive and complex system when you think about it -- you actually have less crit, less hit, less haste as you level -- and it underlies the entire leveling design of the game.
Every time an innovation or new design is incorporated into WoW, every time there's a new ability introduced, a new class, new gear with specific itemization, every time a new system for PvP like PvP Power is introduced it does not merely change itself, or even merely affect the thing it was designed to do. Not only do these changes ripple throughout the game, they perpetuate and propagate and make new design down the road. Resilience went from a flat statistic that reduced your chance to be critically hit to one purely designed for PvP, and necessitating PvP Power's introduction to counteract some of its effects while encouraging PvP gearing as a separate entity from PvE - part of this was caused by players using PvP as a gearing shortcut in previous expansions, which itself was introduced when the original ladder system was reworked. Change begets change, and the game moves forward - the game we will be playing next year was designed looking at the game we're playing now, which was a response to the game we played before.
One of the results of this continuously iterative design is that legacies become enshrined, and interwoven into the game. Ability bloat, as we often call it, is the result of addition without subtraction, and one of the reasons it occurs is because addition feels positive and subtraction feels negative. Make no mistake, we've had abilities removed from the game - Divine Intervention, Shield Bash, Rend - but often those abilities were removed either because there was a competing ability that did the same thing that could be rolled into one, or because it was clearly something that the game didn't need, or at least that designers didn't want to perpetuate. Consolidation can help stem the tide of legacy abilities, but sometimes you just have to yank a spell that doesn't really serve a necessary purpose. Counter to this, however, is the tendency for class homogenization caused by another design change a few years back - the move to try and make 10 man raiding a viable option.
Now, this design change had a lot of positive effects for players - smaller groups could raid, meaning that groups of friends or smaller, cleaner raid groups trimmed down to just a select body of raiders could be established. But in the process, raid design was hampered - you couldn't design a fight for 10 players and assume they would have X ability to counter something you let the boss do. So either you had to design fights with less options at the same time that tanks, healers and DPS were all growing in ways that demanded more options to challenge them, or you had to try and make sure that groups had those abilities. And the only real way to do that was to give those abilities to more classes. It helped make 10 man groups viable, it allowed for raid encounters to be designed assuming some form of those abilities would be present, and it not only contributed to ability bloat (because of course those classes now had a new ability they hadn't had before) but it also meant that those classes lost distinctive identity. Each change propagates through the game, and brings new complexities with it. After nine years, frankly, I'm far less concerned with the big numbers on our gear (the item squish is a solid enough workaround for now) and I'm far more concerned with this complex array of systems designed, implemented and now part and parcel of the game's design.
To a degree, I think the removal of systems like Reforging, the reduction of enchants and gems, as efforts to simplify how gear works is therefore a positive step because it reduces complexity. Complexity can be a good thing, of course, but it has to be controlled. Too many abilities, too many options in terms of how we play, too much to consider in gearing, these can all be equally distracting and worse, must all be accounted for in future design. Homogenization allows raid designers to assume X ability will be present, yes, but it also necessitates it. The design of the fight requires X ability. Someone has to have it.
The more we get, whether it be classes, spells and abilities, stats on gear - the more we get, the more we have. The more we have, the more has to be demanded of us for the sake of challenge - if there's no effort involved, then victory is meaningless, whether it be in killing a raid boss or winning a BG. Without effort there is no fun. A system that forever adds and never subtracts therefore has to constantly account for everything that it has added in order to design for new challenges, and this grows and grows as more is added. As such, the problem goes beyond an item squish, beyond ability bloat on bars, beyond a mass of spell effects displaying at once in a raid, beyond calculating out what combination of arenas and rated BGs will get you the points to buy that upgrade and how it will be affected by the ilevel cap in arenas and rated BG's, because the problem incorporates all of these issues and more. It incorporates every new spell and ability, every new class and every role that class can perform, how each point of rating on each upgrade you get affects your performance. After nine years, World of Warcraft is an unbelievably complex game to design and not much less so to play, if you choose to play it with an eye towards every increment you can squeeze out of it.
The more we get, the more we have. The more we have, the more we have to have in order to meet the challenges designed with that in mind. Sometimes, less is actually more, when less can mean a more meaningful challenge. Sooner or later, World of Warcraft is going to have to cut the complexity down, and that means subtraction. The item squish is one example, and the discussion on ability bloat, the removal of reforging, the change to statistics and the idea of reducing the enchants and gems needed for optimization are all along this line of reasoning - to get more, we need less.